This invention relates to melt processes for the production of clean metals, and, more particularly, to reducing the content of oxides and other low-density substances in the metallic articles made from such metals.
An increasingly important method for the fabrication of metallic articles for critical applications is powder processing. In this approach, fine powder particles of the metallic alloy of interest are first formed. The proper quantity of the particulate or powdered material is placed into a mold or container and compacted by hot or cold isostatic pressing, extrusion, or other means. This powder metallurgical approach has the important advantage that the microstructure of the product produced by powder consolidation is typically finer and more uniform than that produced by conventional techniques. In some instances, the final product can be produced to virtually its final shape, so that little or no final machining is required. Final machining is expensive and wasteful of the alloying materials, and therefore the powder approach to article fabrication is often less expensive than conventional techniques.
The prerequisite to the use of powder fabrication technology is the ability to produce a "clean" powder of the required alloy composition and quality on a commercial scale. The term "clean"refers to an absence of inclusions of foreign substances in the solidified metal. Numerous techniques have been devised for powder production. One of these techniques is the melt atomization process. In the melt atomization process, a melt of the alloy of interest is formed, and a continuous stream of the alloy is produced from the melt. The stream is atomized by a gas jet or a spinning disk, producing solidified particles that are collected and graded for size. Particles that meet the size specifications are retained, and those that do not are recycled through the system for remelting and reprocessing into powder.
When the alloy is melted in a melt vessel such as a hearth prior to atomizing it into powder, an oxide raft typically forms on the surface of the melt, even when an inert atmosphere or vacuum is maintained over the melt. This oxide is present as a result of oxidation of the melt, prior processing of the alloy in ceramic containment vessels, and other reasons. Some or all of the oxide may be swept along with the melt into the atomization apparatus, resulting in the inclusion of oxide particles within, or mixed with, the metallic particles. The oxide particles are processed into the final articles along with the metal, and incorporated into the articles.
The presence of the oxide particles is usually deleterious to the properties of the final articles produced from the powder particles. The oxide particles can either be crack initiation sites or assist in crack propagation, leading to premature failure of the article. Since the oxide particles cannot be readily removed from the powder mix or the articles, it is important to prevent the oxide from entering the atomization process in the first place.
There are two possible approaches to preventing oxides from entering the final articles. One is to prevent or control the formation of oxides or oxide rafts, and the other is to permit the oxides or oxide rafts to form, but to prevent the oxides from reaching the atomizer.
Various techniques such as atmosphere composition control have been used in an attempt to prevent formation or cause reduction of the oxide in the first place, but the thermodynamics of oxide formation dictates that the oxides can form even in the presence of very small oxygen contents. Atmosphere control to reduce oxides is, in many cases, impractical because of its adverse effects on the overall production operation and costs, and on the final product.
Another approach is to permit oxides or oxide rafts to form, and then prevent it from reaching the powder. Since oxides and other types of ceramic impurities have densities that are less than that of the metallic alloys that are melted, they float on the surface of the melt typically as agglomerated rafts of particles. In one such technique, the surface-applied heat source is used to "herd" the oxide rafts away from the pouring spout of the hearth, reducing the likelihood that oxide will pass through the spout to the atomization process. The rafts can be herded behind dams placed across the metal surface.
Although herding of the oxide rafts has met with some success, such herding becomes progressively more difficult as additional oxide forms during the melting process. Various techniques have been tried to periodically skim the oxide rafts from the surface of the melt, but these have not been entirely successful. Oxide inclusion in powders prepared by melt atomization remains a problem, particularly for extended powder production runs.
The problem of cleanliness of the molten metal has been discussed in relation to powder production. However, the same problem arises in relation to the preparation of ingots of high cleanliness. There is therefore a need for a better approach to preventing oxides from being incorporated into molten metals. The present invention fulfills this need, and further provides related advantages.